window

N lanes of a followed by b, starting at lane start: window(a, b, 0) is a, window(a, b, N) is b. With a the previous block of a stream and b the current one, window(a, b, N - k) is the current block moved k lanes later, with the end of the previous one in front.

window :: (a: Vector($T, $N), b: Vector(T, N), $start: int) -> Vector(T, N)